This invention relates generally to a novel method of seismic exploration and more particularly to a method for surface seismic exploration designed to eliminate or compensate for near surface effects tending to degrade the effectiveness of data acquisition.
The seismic signal resulting from actuation of a seismic source at the surface of the earth undergoes a filtering action as it passes through the earth and is stretched out into a so called "wavelet". Reflection of such wavelet from successive subterranean rock boundaries produces a great number of additional wavelets which may be detected and recorded as a seismic record or seismogram. The recorded wave form or "signature" of each such wavelet is said to constitute the successive convolution of the seismic signal with the various portions of the earth through which the signal or its various wavelet modifications travel. As used in the discussion to follow the term "source signature" shall mean the shape or characteristics of the down going seismic signal at any point along its path. By "primary source signature" shall be meant the signature of the seismic signal before experiencing any filtering effects.
Since the seismic signal returns from deep reflectors as a series of wavelets instead of a series of sharp pulses of very short time duration ("impulses") a seismogram consists of a whole series of overlapping wavelets. What is desired is the echo sequence or impulse response of the earth uncontaminated by the wavelet. In so called "spiking deconvolution" a filter is designed which, when applied to the seismogram removes or "collapses" the wavelet and reveals the echo sequence itself. Techniques for the design of such a spiking filter are well-known to the art. But in order to practice such technique, one must not only make assumptions concerning the properties of the echo sequence but one must either know or make assumptions concerning the characteristics of the wavelet, i.e., the source signature. To the extent that this process requires making guesses or predictions about the nature of the source signature, it is inherently flawed. This problem is clearly outlined in Ziolkowski, Deconvolution, copyright 1984 by International Human Resources Development Corporation, Massachusetts, pages 1-12, the contents of which are incorporated herein by reference.
The explorationist is not dependent entirely upon guesswork, however, in the design of a spiking filter. For example, in marine exploration it is possible to measure the characteristics of the signal generated by a marine source utilizing a signature calibration test. Here the pulses are measured well away from the source and far enough from the sea bottom to avoid bottom reflections. Such a signature test reliably reveals the source signature because water is a homogeneous medium. Similar measurements are more difficult on land but have been attempted. For example, surface measurements of earth motion generated by vibratory seismic sources have been performed with the aid of velocity meters attached to vibrator base plates. Standard geophones have also been implanted in the earth's surface immediately adjacent various types of seismic sources. But spiking deconvolution utilizing a source signature obtained by such calibration tests, so-called "signature deconvolution", is hampered by the fact that these signatures are not those of the down going wavelet which is seen by the deep reflectors of interest. This is because the wave form of the source undergoes substantial alteration or filtering as it passes through the the earth's near surface layers.
The most important of the near surface layers is usually termed the "weathered layer" or "low velocity layer", often abbreviated "LVL". The LVL lies immediately beneath the surface and includes the topsoil, subsoil, and partially disintegrated or unconsolidated rock. The LVL often varies in thickness, density, lithology, velocity, and attenuation effects.
Various methods have been employed for correcting seismic reflection records to compensate a seismic record for the influence of the LVL involving estimates of signal transit time and frequency dependence for example as determined by an uphole survey. However, near surface effects are frequently much more complicated than can be accounted for through analysis of a small number of uphole tests or by the often used assumption of a simple uniform velocity layer of variable thickness. Consequently, none of the existing near surface corrections presently employed are completely successful.
A general objective of the present invention is to provide a novel method of seismic exploration involving acquisition and processing of seismic records of greater accuracy utilizing improved corrections for near surface effects.
A still further objective of this invention is to devise a method of seismic exploration wherein the source signature utilized in the design of a spiking or signature deconvolution filter is more accurately characterized.